Facebook has been criticised by a gun safety group for taking millions of dollars in advertising revenue to promote a legal "loophole"
It allows Americans to obtain "concealed carry" weapon permits without any physical training.
The social network's own records show at least US$3.7 million has been spent since May advertising the "Virginia loophole".
It enables would-be gun carriers to get a permit in a few minutes by answering 10 questions on their mobile phone, without ever touching a firearm or meeting an instructor.
A company promoting the controversial scheme has been designated a "political advertiser" by Facebook and has become its third biggest spender, after Donald Trump and Beto O'Rourke, a leading Democrat.